1. Field of the Invention
This invention relates to electronic engine control and, more particularly, to a method and system for controlling ignition timing of an engine with dual independent camshafts.
2. Background Art
Variable cam timing systems, such as that described in U.S. Pat. No. 5,117,784 to Schechter et al., assigned to the assignee of the present invention, operate to vary the timing between the camshaft and the crankshaft to optimize engine performance over the entire range of engine operation. In my copending patent application, FMC 1286 PUS, incorporated herein by reference, a variable cam timing system is provided for an engine equipped with dual camshafts that permit the intake and exhaust cam timing to be controlled independently. Since cam timing greatly affects many aspects of engine control, care must be taken in both the control of, and reaction to, the requested timing. For example, spark for maximum best torque (MBT) and borderline spark (BDL) must be varied with valve timing in order to determine the optimal spark timing. Accordingly, there is a need for an ignition spark timing method for an engine with dual independent cam timing that optimizes control system behavior while minimizing complexity.
It is an object of the present invention to provide a method and system of ignition spark timing control for an engine having dual independent camshafts. It is another object of the present invention to provide a method and system for substantially reducing the ignition timing calibration effort while avoiding a proliferation of ROM lookup tables.
In accordance with a preferred embodiment of the present invention, MBT ignition timing calibration values and BDL ignition timing calibration values are stored in stability limited (SL) and optimum power (OP) calibration tables. Optimum MBT and BDL ignition timing values are then determined using a performance or power index (PI) value that is indicative of the driver""s relative desire for fuel economy or power. The PI value is used to interpolate over the range of values in the SL tables to the values in the OP tables to calculate values of MBT and BDL that are optimal for all altitudes and levels of driver demand. The ignition timing value used at any point in time is preferably the lesser of the MBT value and the BDL value. The calibration values for MBT and BDL ignition timing are coordinated with calibration cam timing values so that optimal ignition timing as well as cam timing will be produced over the range of engine operation.